The invention relates to a control unit for setting the transmission ratio of a transmission which is arranged in a drive train together with an engine and to a method for operating such a control unit.
It is known, for example from the German laid-open application DE 42 30 101 A1, to generate a predefined braking torque in a motor vehicle, on the one hand by means of the braking torque of a service brake and on the other hand by means of the braking torque (drag torque) of the engine, the service brake being relieved of loading, which is advantageous in particular in the case of long-lasting braking operations as otherwise there is the risk of the service brake being overheated and losing its optimum braking effect.
German laid-open application DE 43 30 391 A1 also discloses a control unit for a drive train of a motor vehicle in which a desired braking effect is determined from the activation of the brake pedal, which braking effect in turn leads to shifting down of the automatic transmission in order to increase the braking effect of the engine. Here, use is made of the fact that the braking effect of the engine rises as the rotational speed increases so that at a predetermined speed the braking effect created is greater in a low gear than in a higher gear.
A disadvantage of the known arrangements described above is however the fact that the braking torque of the engine increases with the rotational speed so that, in order to achieve a good braking effect of the engine, relatively high rotational speeds are necessary, which is problematic in terms of noise, wear and comfort.